The present invention relates to a transmitter and a method of operating a transmitter, and in particular to a dual band radio transmitter and a method of operating a dual band radio transmitter.
In order to overcome problems concerning the lack of channel capacity for portable communication devices such as cellular mobile telephones, there is a requirement to produce devices capable of communication in two separate radio bands. In Europe, there is a drive towards so-called dual band devices operating both at 900 MHz (GSM) and at 1800 MHz (DCS). Such dual band devices require a corresponding dual band transmitter. These dual band transmitters will typically have a lower band transmit path (e.g. for passing 900 MHz signals) and a higher band transmit path (e.g. for passing 1800 MHz signals). One difficulty faced by dual band transmitters is that when they are transmitting at the lower band frequency (e.g. 900 MHz) unwanted harmonics of tne lower band frequency may be coupled to the higher band transmit path; additionally, coupling of the lower band signals to the higher band transmit path may cause the higher band transmit path to generate unwanted harmonics of the lower band signal in addition to those generated in the lower band transmit path such that even a perfectly linear lower band transmit path (which generates no unwanted harmonics itself) could still give rise to an unacceptably large amount of energy being transmitted at the second or higher harmonic frequencies of the lower band signal due to regeneration in the higher band transmit path.
Note that this problem arises where the second or higher harmonics are close to or less than the frequency of the higher band (since otherwise a passive low pass filter can be used to remove any signals of a higher frequency than the higher band frequency which will filter out second or higher harmonics from the lower band transmit path). In the case of GSM and DCS dual band transmitters, the problem is especially acute since the second harmonic of the lower band is exactly equal to the frequency of the higher band (1800 MHz).
There are a number of conventional dual band transmitters which address this problem. One particularly simple and robust solution is to provide two separate transmit paths (including separate power amplifiers and separate passive filters) which are combined together at the antenna, with sufficient rf shielding between the two paths to provide at least 50 dB of isolation between the two paths. The disadvantage of this solution however is that it is very space inefficient and is therefore undesirable for modern portable communication devices.
An alternative known solution in which there is no significant shielding between the lower and higher band transmit paths (thus enabling them to be formed within a single package) requires the use of a switchable low pass filter. Note, however, that even without shielding, the leakage from one transmit path to the other will still be relatively small (e.g. there will be about 30 dB of isolation between the paths), but this must still be considerably reduced to comply with most telecommunications standards (e.g. the GSM specification requires that any unwanted harmonics are at least 70 dB lower than the transmitted signal). The switchable low pass filter may be configured either as a higher band low pass filter or as a low pass filter incorporating the rejection of the lower band second harmonic within the higher band transmit path. In a similar alternative known solution, there is only a single transmit path including a switchable power amplifier which may be switched between operating at the lower band and the higher band together with a switchable low pass filter as before. The basic structure of a low pass filter is a capacitor connected in parallel with the output of the power amplifier (i.e. between the transmit path and ground) for shorting high frequency signals, and an inductance in series with the output of the power amplifier, for choking high frequency signals along the transmit path. Such a filter has a corner frequency which inversely depends upon the square root of the inductance of the filter and the square root of the parallel capacitance of the filter. Thus, in order to decrease the corner frequency of the low pass filter (e.g. to prevent the transmission of the lower band second harmonic when operating in the lower band mode) one could either increase the capacitance to allow lower frequency signals to be shorted, or increase the inductance to provide greater impedance to lower frequency signals traveling along the transmit path.
The basic known method for increasing either the capacitance or the inductance is to use a diode as an on/off switch, the state of which is controlled by a dc bias voltage, to switch an extra capacitor or inductor into or out of the filter circuit. Where the total capacitance of the circuit is to be controlled, the diode is placed in series with an additional capacitor; when the diode is switched on the additional capacitance is included in the filter and the corner frequency of the filter is lowered. This has the disadvantage that the diode needs to be switched off when the transmitter is operating in the higher band, and this requires that a large negative bias voltage be applied to the diode to maintain the diode in an off state when large rf signals are being passed through the switchable filter. This is disadvantageous because a negative voltage generator needs to be provided just for this purpose (which increases the cost of the filter).
In the case of the inductance, an extra inductor can be switched into and out of a circuit by providing a by-pass (short circuit path comprising ideally just a diode although extra components such as decoupling capacitors, bias voltage generation circuitry and associated r.f. chokes etc. will also be required) in parallel with the extra inductor, which by-pass can be switched off by switching off the diode switch contained within the by-pass. This has the advantage that the diode is switched off when the transmitter is transmitting in the lower band mode so if there are two separate transmit paths with some degree of shielding between them, not such a large negative bias is required to maintain the diode in an off state (and such a voltage may already be available to the filter without requiring a dedicated voltage generation circuit). However, this arrangement is complex, has a heavy reliance on inductors which are difficult to manufacture accurately and most significantly, this solution is undesirably lossy (as a result of the resistance of the diode through which the entire wanted signal must flow) causing a loss of approximately 1 dB. of the r.f. signal when operating in the higher band.
According to a first aspect of the present invention, there is provided a dual band transmitter including a power amplifier having an output, a stub having a first and a second end and a switch which is switchable between a first and a second state, wherein the first end of the stub is coupled to the output of the power amplifier and the second end of the stub is coupled to the switch, whereby the second end of the stub may be substantially closed or opened in dependence upon the state of the switch.
The term stub is well known in the art and refers to an element which is capable of supporting standing electrical waves at particular resonant frequencies in dependence on the properties of the stub. For example, in longwave radio applications, stubs are typically formed from co-axial cables. However, in the microwave realm with which the present invention is more concerned, a stub may be more conveniently formed from a pair of parallel conductors with a suitable dielectric located therebetween (e.g. by printing one or more conductive tracks onto a printed circuit board substrate). Of course, other possible ways of constructing a suitable stub will be readily apparent to a person skilled in the art; for example, a suitable arrangement of discrete or distributed capacitors and inductors could be used (in accordance with well known transmission line theory, etc.).
Furthermore, it is well known that stubs can be used as fairly narrow-band notch filters (note that stubs cannot readily be used to build wide-band filters such as low-pass filters as required in a radio transmitter). Transmission line theory dictates that the frequency at which the notch (of a notch filter built from a stub) occurs depends on the length of the stub in terms of the wavelength of electrical excitations propagating through the stub and on whether the distal end of the stub is open or closed. Typically, for two stubs identical in all respects except that one has an open end and the other a closed end, the stub having an open end would prevent the passage (along a given transmit path) of electrical signals having a frequency half as large as those which would be blocked by the stub having a closed end if both stubs were connected in the same way to the transmit path. Note that in the present context, the term closed end refers to the second end of the stub being connected to ground (or more generally to the other conductor of the stubxe2x80x94this is usually a ground plane) such that the voltage at this point is forced to be constant (at ground) by analogy with an acoustic pipe with a closed end since no vibrations are possible at a closed end of an acoustic pipe and thus nodes are forced at the xe2x80x98closed endsxe2x80x99 (of both the stub and the acoustic pipe). Similarly, the term opened end refers to the case where the end of the stub is floating since this is analogous to the case of an acoustic pipe having an open end (note that anti-nodes will be formed at open ends when resonance is occurring).
Preferably, the dual band transmitter comprises a lower band power amplifier for operating at a lower band frequency; a higher band power amplifier for operating at a higher band frequency; a lower band transmit path between the output of the lower band power amplifier and a combining means; and a higher band transmit path between the output of the higher band power amplifier and the combining means, wherein the stub is connected to the higher band transmit path. Preferably, the lower band transmit path incorporates a lower band low pass filter for removing all signals having a frequency greater than or equal to the second harmonic of the lower band wanted signal, and the higher band transmit path includes a higher band low pass filter for removing all signals having a frequency greater than or equal to the second harmonic of the higher band wanted signal.
The present invention gives rise to a significant advantage over conventional arrangements because the stub provides a very efficient switchable filter which can be tuned to take out the second harmonic of the lower band power amplifier when the lower band power amplifier is operating, without the associated switch regenerating further harmonics itself because it is in an off state (when it is most likely to cause regeneration) only when the lower band power amplifier is operating (and thus only small currents are flowing in the higher band transmit path) and because when it is in an on state, the wanted signal does not need to pass through this switch. Thus an active switch (such as a diode) may be used without causing any of the disadvantages usually associated with using an active switch in a transmit path. The fact that the stub only provides very narrow-band notch filtering is not a problem in the case of a radio communication system such as GSM where the wanted transmitted signal only occupies a relatively narrow frequency band, and where most of the noise generated by the power amplifier occurs only at harmonics of the wanted signal.
Preferably, the switch is formed by means of a relatively simple PIN diode which can be switched into an on state by the application of a forward biasing positive dc voltage, or into an off state by removing all dc bias from the diode.
The present invention gives rise to the significant advantages over the conventional arrangements discussed above of not requiring a large amount of shielding between the transmit paths, not requiring a large negative voltage generator (since the diode will be off only when only small currents are flowing in the higher band transmit path) and of giving rise to only a very small loss of signal strength (e.g. 0.2 dB).
According to a particularly preferred embodiment of the present invention, the transmitter is a dual band transmitter in which the higher band is twice the frequency of the lower band, as is the case, for example, in a GSM/DCS dual band transmitter. Such an arrangement is particularly advantageous because the stub will act to at least partially filter out the second harmonic of the higher band signal when operating in the higher band because a closed end stub is approximately equivalent to an open end stub of half the length (thus acting as a notch filter with the notch at twice the frequency). In order to compensate for any parasitic capacitance of the switch, the switch preferably includes suitable compensation means. According to one preferred embodiment, the compensation means includes a small negative voltage bias supply to the switch for reducing the parasitic capacitance of the switch when in an off-state. According to another preferred embodiment, the compensation means includes tuning elements in the form of further passive capacitors and/or inductors. Alternatively, a switch can be used which has a very low parasitic capacitance even when it is unbiased, preferably of less than 0.3 pF at 0V bias.
According to a second aspect of the present invention, there is provided a surface mounted device for use in a oual band transmitter, comprising a stub having a first end for connection to a higher band transmit path of the dual band transmitter and a second end, and a switch having a first terminal for connection to a bias voltage supply terminal and to the second end of the stub and having a second terminal for connection to a fixed reference voltage, wherein the switch comprises a diode whose state may be varied between an on state and an off state by supplying either a positive voltage or a substantially zero or negative voltage to the first terminal of the switch. Preferably the device includes a resistor having a first terminal for connection to the first terminal of the switch and a second terminal for connection to the bias voltage supply terminal whereby the bias voltage supply terminal may be connected to the first terminal of the switch via the resistor. The device may further include an inductor connected in series with the resistor whereby the bias voltage supply terminal may be connected to the first terminal of the switch via both the resistor and the inductor.
The stub is preferably printed on a small piece of ceramic.